1. Technical Field of the Invention
The present invention relates to the preparation, via anionic polymerization, of polymethyl methacrylate ("PMMA") polymers having a relatively high content of syndiotactic triads, such high content of syndiotactic triads therein imparting to the final polymers a higher glass transition temperature and, therefore, greater resistance to heat.
2. Description of the Prior Art
It is known to this art to prepare polymethyl methacrylate via a free-radical technique. However, the polymers obtained have glass transition temperatures in the range of 100.degree. to 110.degree. C., which corresponds to contents of syndiotactic triads in the range of 60% to 65%.
These polymers have also been prepared by the anionic technique in the presence of an initiator system comprising an initiator and a ligand. The presence of a ligand is generally necessary to prevent side reactions due to the attack, by the anions, of the ester groups of the methyl methacrylate monomer.
Organic compounds of aluminum are efficient ligands, used in combination with conventional organolithium initiators, to promote the anionic polymerization of methyl methacrylate in toluene. Exemplary of these ligand compounds are the trialkyl aluminums and the dialkyl phenoxy aluminums.
Ligands of the AlR.sub.3 type (R=unhindered alkyl) have long been employed to prepare a highly syndiotactic polymethyl methacrylate. For example, Hatada et al, Makromol. Chem., Suppl. 15, 167 (1989), report that an initiator system tBuLi/Et.sub.3 Al (mole ratio initiator/ligand greater than 1/3) produces a polymethyl methacrylate having over 90% of syndiotactic triads, in toluene at -78.degree. C. The drawback of the ligands of this type is that the polymerization must be carried out at very low temperature.
R.sub.3 Al ligands have also been used, but containing bulky R moieties (such as di- or tri-isobutyl), requiring polymerization temperatures of about 0.degree. C. The polymers thus obtained, described in EP-A-0,434,316 exhibit, in particular, a value of Tg in the range of 112.degree. C. to 119.degree. C., indicating a syndiotacticity approaching that of the free-radical polymethyl methacrylate, i.e., contents of syndiotactic triads on the order of 60% to 65%.
Haddleton et al, Polymer Preprints, 34, 564-565 (1993), describe a syndiotacticity of polymethyl methacrylate in the range of 33% to 67% employing a tBuLi/tri-isobutyl aluminum system at ambient temperature.